Fire detectors



Jan. 7, 1964 F. c. LEMAIRE 3,117,311

FIRE DETECTORS Filed June. 19, 195i 4 Sheets-Sheet 1 Fig.2

- o 20 6o t(c) Jan. 7, 1964 F. c. LEMAIRE 3,117,311

FIRE DETECTORS Filed June 19, 1959 4.Sheets-Sheet s F. c. LEMAIRE3,117,311

Jan. 7, 1964 FIRE DETECTORS Filed June 19, 1959 4 Sheets-Sheet 4 j i .D7'71 70 A11. ARM PEA/ Y INVEN TOR.

ATTORNEYS Fem/s c. A swam/s United States Patent 3,117,311 FIREDETECTORS Frans C. Lemaire, 115 Rue du Cuirasse, Ghent, Belgium FiledJune 19, 1959, Ser. No. 821,442 Claims priority, application BelgiumJune 21, 1958 5 Claims. (El. 340233) The present invention relates ingeneral to electronic fire detectors and in particular to an alarmcircuit for detecting a predetermined rate of temperature rise or apredetermined temperature rise.

Fire detectors can be based'on one of two criteria the thermometriccriterion, or the exceeding of a critical temperature considereddangerous, or the velocimetric criterioni.e., an excessively rapidrising in the temperature of the area to be supervised. Apparatus basedon the first criterion usually make use of the expansion of atemperature-sensitive member which is used similarly to the expandingsubstance of a thermometer. A temperature change therefore manifestsitself mainly by a deformation or an increase in volume leading to thephysical movement of a mechanical member or to the opening or closure ofa contact. Such an apparatus must not trip below a minimum temperature,otherwise it will set off alarms accidentially.

Whatever the physical construction of apparatus of the kind specified,the thermometric criterion is of doubtful value, because the time takento reach the required critical temperature varies considerablywith theinitial temperaturei.e., the normal temperature. Such detectorstherefore usually provide secondary supervision.

Velocimetric apparatusi.e., apparatus sensible to the rate oftemperature rise-are more reliable for they respond to a rate ofincrease and are therefore basically independent of normal temperatureconditions. As a rule, velocimetric detectors use the same elements asthe absolute detectors but the temperature-sensitive elements aredoubled and, although mounted close together, are differentiated bytheir thermocoupling with the surroundings to be supervised. Onesensitive element is directly associated with the surroundings, whilethe other sensitive element is suitably isolated therefrom. Hence, ifthe ambient temperature rises sufficiently rapidly, the temperature ofeach sensitive element follows a different pattern. The delay in thetemperature of the isolated element becoming adapted to the new ambienttemperature is greater in proportion as the latter element is betterprotected and as the temperature rises more rapidly. If this differencein thermal adaptation can be made obvious, .a velocimetric detector isprovided. Slow temperature variations will reduce the difiere'ncesbetween the temperature of the elements, the difference between thephysical conditions of the elements will therefore be negligible, and nowarning signal will be given.

Sensitive elements such as solids, springs, filaments, strips, etc., orliquids such as mercury, alcohol, etc., or gases such as air, nitrogen,etc. may be utilized.

Temperature differences between solids are indicated by the relativemovement of members connected to such solids. When liquids are employed,vessels are used which communicate with one another and which areinsulated differently from the external surrounding medium. The vesselscontain a liquid which vaporizes at ordinary temperatures, and due tothe slight temperature difference when the; branches are externallyheated, the difference in insulation leads to a difference in vapourpressure. The difference in vapour pressure in turn leads to adifference in liquid level, with the result that an electric contact isoperated.

Similarly, with gases, two seal-tight enclosures filled with gas, suchas air, nitrogen or the like, are also difierently insulated from thesurroundings. A rapid temperature variation leads to a pressuredifference operating, for instance, a differential manometer which, inturn, operates a mechanical member or an electric contact.

Electrical apparatus operating along the same lines consists of acircuit which comprises one or more temperature-sensitive elementsconnected into one arm of a Wheatstone bridge, a similar number ofidentical electrical elements being placed in' the opposite arm.'However, the last-mentioned elements are thermally insulated from thesurroundings or have an elevated heat inertia, for instance, by virtueof their mass. When the temperature varies fairly slowly, both systemsvary similarly for the same temperature rise, but their resistancediffer from one another when the temperature rises rapidly. In the firstcase, the bridge remains substantially balanced, while in the secondcase an out-of-balance voltage appears which can be used as a warningsignal.

In other apparatus, temperature variations are applied to a chamberexposed to the surrounding medium. The

temperature variations lead to an excess pressure being applied to acontrolled aperture valve, so adjusted that the excess gas associatedwith reduced rates of temperature variation can escape to atmosphere orto a compensating chamber (apparatus comprising a compensated tubularaerothermal chamber), with the result that there is scarcely any rise inthe pressure in the main chamber.

However, rapid temperature variations lead to a considerable excesspressure which cannot be neutralized by the valvenotwithstanding theincreased rate of gas fiow therethrough. The excess pressure operates asuitable pressure gauge which in turn operates a control member orelectrical contact.

Apparatus of the kind specified can be highly sensitive, but highsensitivity is achieved at the cost of stability, for in very sensitiveapparatus it is difficult to provide a stable boundary between theoperative and the inoperative states.

Also, high mechanical precision must be embodied in expansion apparatus,since the force applied by the expanded elements are reduced asignificant amount. An inevitable result of mechanical precision is highsensitivity to shocks and atmospheric agents such as salt air, dusty orcorrosive atmosphere, dampness, etc. Similar considerations apply toapparatus having progressive escape valves, in which accidental blockageoccur which are often invisible and, for that reason, all the moredifficult to locate. When intended for use on' ships, such apparatus isusually provided with an anti shock mechanical suspension and madecompletely seal-tight. As a result, sensitivity is usually reduced,dimensions are incerased and the appearance is not very aesthetic.

Finally, apparatus using bimetallic strips, trimetallic strips,filaments, springs, diaphragms, etc., often goes out of order due to aslow change in the nature of the metal (aging, recrystallization, creep,slow deformation due to the variation of internal strains) or to a lackof seal-tightness.

Furthermore, if there is a considerable and rapid drop in temperaturethe very nature of vel'ocimetric detectors prevents them fromimmediately adapting themselves to the new heat conditions. Hence, whilethey are adapting themselves their velocimetric sensitivity is bound tobe reduced. This case occurs, for instance, when the place to beprotected is suddenly subjected to an outside winter temperature(opening of the doors of hangers, sheds, etc.).

Detectors in which the temperature gives rise to purely electricaleffects in the sensitive element form a class apart. Amongst suchdevices are thermo-couple devices; the pot and cold junctions aredifferently insulated from the surroundings so that fairly rapidtemperature 3 variations produce a potential difference in the circuitdue to the behaviour of the junctions, and the potential difference isused as a warning signal.

Another device uses a thermistor supplying a relay and is based on thefact that under certain circumstances the thermistor may have a negativeresistance. When cold the thermistor operates in the linear part of itsvoltagecurrent characteristics; its resistance is high and the currentis very low. When the temperature rises above a certain level, theresistance of the thermistor decreases and the current rises, and thisstate of affairs is amplified due to dissipation by Joules effect. Thethermistor finally operates in the negative part of its characteristicand the current rapidly rises to the value required to trip the warningrelay.

The novel electronic detectors to be described hereinafter have all thepossibilities of conventional detectors but, due to the inherentflexibility of their circuits, they can be readily adapted to specialdetection requirements. This applies not only to giving a warning butalso to the immediate indication of faults in the detectors themselves.

To show the novelty of the operating principles and properties of theapparatus according to this invention, a survey will be made of theoperational role of various elements of conventional detectors.

Some known velocimetric devices give a warning on the basis of acomparison between an inoperative state, when the temperature of thesensing element or elements is stationary, and a new thermal conditionassociated with the start of a fire and distinguished by a temperaturerising at an excessive rate. This comparison implies that asemi-permanent memory element is always available which at any givemoment records the thermal conditions during a specified and immediatelyprevious period of about 30 to 60 seconds or several minutes, whilegradually becoming adapted to the new thermal conditions which areslowly being established. In most apparatus this memory element takesthe form of a second temperaturesensitive sensing element which doublesthe actual supervisory sensing element. However, the thermal change ofthe secondary sensing element is delayed as compared with the thermalchange of the main element because the secondary element is notsubjected to heating in the same way as the main element. The delay isproduced either by additional thermal insulation or by appropriatelyincreasing the thermal mass of the sensitive element. The arrangement ofsensitive elements is so contrived that when the difference between thethermal changes of the elements reaches a value considered excessive,such difference produces an electrical or mechanical effect which can beindicated and fixed. On the other hand, when the heat change of thesupervised area is slow, the thermal delay of the secondary element isreduced and cannot cause any noticeable effect.

Other detectors comprise only one sensitive element so arranged that thephysical state and sensitive characteristics of the element (as a rule,the pressure of a gas or of air) are continuously adapted to slowtemperature variations, whereas with rapid temperature variations thereis an increasing delay in adaptation leading to a physical change whichcan be demonstrated.

In the electronic devices to be described hereinafter the compensationfor a slow temperature increase is provided by the slow discharge of acondenser, the electric circuit not delivering any signal. However, whenthe rate of temperature increase exceeds a value considered excessive,the condenser discharge rate becomes inadequate and a warning signal isgiven. Hence the condenser and an associated discharge circuit form thesemi-permanent memory element hereinbefore described. As will bedescribed hereinafter, the condenser also acts as an element fordecoupling and concentrating a number of detector circuits in the sameindicating circuit.

The electronic detectors to be described hereinafter are mixed detectorshaving both thermometric and velocimetric sensitivity. If required,however, they may have only one kind of sensitivity. If they havevelocimetric sensitivity, thermal compensation for slow heat changes isprogressive and is provided by the slow discharge of the condenser.

The detectors according to this invention each comprise only one sensingelement which can, however, be subdivided into a number of unitsdistributed in the area to be supervised. The sensing element comprisesa temperature-sensitive element associated with a heat collector. Sincethe mass of the sensitive element is grealy reduced, an assemblycomprising a sensing element and a collector is no bigger than thediameter of a large coin.

The present novel detector is distinguished by the follOWing exclusivefeatures:

(1) The sensitive element, hereinafter referred to as the sensingelement, is completely static. It is of very small dimensions, has lowthermal inertia and consists of a substance insensitive to aircomposition and to dust. The heat collector to which it is secured canbe of reduced dimensions. Detector elements can therefore be of discreteappearance yet have the required sensitivity; also, the sensing elementis completely unaffected by shock, vibration, dust and the ambientatmosphere (detectors are subjected to dust in industrial installationsand to salt air on board ship).

(2) The sensing element associated with the electronic indicatingcircuit can have either a thermometric or a velocimetric sensitivity orboth, simultaneously, yet independent control thereof is possible.

(3) The semi-permanent memory element providing gradual compensation ofa slow temperature rise is a condenser, the charge of which continuouslyadapts to the temperature of the sensing element, providing that thetemperature rise thereof is slow. However, the critical value of thevariation rate can be adjusted as required. it rapid drop in temperatureis compensated without de- (4) As normally provided, the sensing elementis of the point kind, although a number of point elements can beconnected in series or parallel in the same circuit. When distributedover the area to be supervised, the sensing elements are the equivalentof a compensated tubular aerothermal detector.

(5) Substantially any number of circuits of the kind just described canbe connected to a single centralized warning installation.

(6) The aforesaid compensating condensers not only provide compensationbut also act as decoupling elements by blocking the DC. component of thevoltage delivered by each of the circuits with which the condensers areassociated.

(7) Although a number of detector circuits are concentrated on onecentral warning device, the velocimetric and thermometric sensitivity ofeach detector circuit can be individually adjusted, as required, withinwide limits.

(8) Any circuit giving a warning is identified through the agency of thecentral warning device. The identified circuit appears on a centralpanel as an individual light signal; also a permanent general acousticwarning is operated. Finally, the useless detector circuit iseliminated.

(9) When a detector circuit or one or more sensing elements thereofceases or cease to operate, even for a very brief time, a warning otherthan the fire warning is given, the faulty circuit being identified,marked and eliminated as in the previous case. These operations take ashort fraction of a second.

The central warning circuit is immediately made available again to theunaffected detector circuits.

(10) The current consumption of the detector circuits and warningcircuit is greatly reduced, hence the size of the supply batteries, andthe capacity of the charger therefor reduced a substantial amount.

The detectors to be described hereinafter comprise a number ofsemi-conductor sensing elements, installed in groups or individually,sensitive to the temperature of their surroundings. The elements areelectrically connected to an indicating or warning device whichinterprets the signal from the sensing elements and considers, asrequired, the rate of variation or the absolute amplitude of suchsignals or both such rate and such amplitude. The Warning deviceprovides a permanent warning'signal and identifies the detector circuitwhich has tripped. Also, the warning circuit provides constantsupervision of the state of operation of each circuit, and for anytechnical defect, and identifies the faulty circuit.

Any sensing eleemnt can be adapted to respond either to an excessiverate of temperature rise or to an absolute temperature or to both. Thevalues which are considered to be critical for such rates and absolutetemperatures can be set individually in each sensing element or sensingelement group associated with a given detector circuit.

The invention will be fully understood from the following descriptionand the drawing in which:

FIG. la is a top view of a temperature sensing element;

FIG. 1b is a sectional view taken along the line X-X, YY of FIG. 1a.;

FIGS. 2, 44, and 5 are graphs of the characteristics of the temperaturesensing device;

FIGS. 3, 6 and 17 are diagrams of the temperature sensing circuit;

FIGS. 7 to 9 and 11 are graphs explanatory of the operation of thecircuit of FIG. 6;

FIG. is a diagram of the amplifier and relay portion of the circuitaccording to the invention;

FIGS. 12 to 16 are circuit diagrams of various embodiments of theinvention.

A description will hereinafter be given of the various operationallyseparable elements of the electronic detector of this invention, asfollows:

(1) The sensing elements and the corresponding sensing circuits;

(2) The differentiating circuits associated therewith to form detectorcircuits;

(3) The simple indicating circuit associated with the precedingcircuits;

(4) The indicating circuit associated with several grouped detectorcircuits jointly;

(5) The identification and marking circuit.

I. Sensing Elements-Sensing Circuit These mainly comprise atemperature-sensitive electrical conductor or semiconductor elementsecured to a suitable heat collector. An example of a. sensing elementhaving a semi-conductor body Th and a collector C of thin sheet copperor aluminium is illustrated in FIGS. 1a and 1b. Of course, otherphysical construction bringing together the same operating elements canbe considered and therefore fall under the invention.

More particularly, the sensing element for supervising temperaturevariations of a solid such as, for instance, a machine bearing or anelectric cable, come within the same classification, for in such a casethe solid to be supervised is thermally connected to the sensitiveelement, for instance, by a plate or flange secured to or around thesolid.

It will be hereinafter assumed that the sensitive element is a negativetemperature coefiicient resistance, for instance, a thermistor. Ofcourse, sensitive elements having, for instance, a positive coefiicient,.can be used without departing from the scope of the invention. Thecircuits can be adapted accordingly.

The relationship between the absolute temperature T of the thermistorand its resistance Th can be stated by the formula:

Th-=Ae X%= (in per degrees Kelvin) (2) The thermistor isconnected inseries with a resistance and battery to provide a sensing circuit (FIG.3).

'It will be assumed that the current flowing through Th is neversufiicient to raise the temperature thereof much above ambienttemperature.

If the temperature is varied by a value dT (or dt) from a value at whichTh equals R, it can readily be shown that the elementary variation ofthe potential of the point M (FIG. 3) is:

ail ODOZSaEdt volts/ degree =Xdt volts/ degree where X represents theelementary signal corresponding to aone degree temperature variation.

If the same circuit is now considered to be at a temperature at which Thno longer equals R and if K: Th/R, the elementary signal obtained is:

KX t M d The function (K) has the same values for a given value of thevariable and its reciprocal. Hence the curve f(K) shown as following alogarithmic scale for K is symmetrical about the abscissa K=I (curve 1of FIG. 4).

A value of R will be assumed such that K is unity for the meantemperature of the temperature range under consideration. It will benoted in particular that, if K lies between 0.38 and 2.6, the functionf(K) varies by not more, than 10% around its mean value in thisinterval. This means that, with an appropriate choice of temperaturecoefiicient, detector sensitivity can be constant to within in thisinterval. For instance, if X=3.3%, the interval extends over 60, forinstance, from -15 to +45 In fact, the coefiicient or is not .constantand its variation is less negligible in proportion as the temperature islower; It can be shown that the sensitivity curve will therefore be thecurve 2 shown in FIG. 4 which has substantially the same temperaturerange between its end values hereinbefore describedi.e., between K and IThe value of R will be so chosen that at the mean temperature of therange to be considered, the value of K will be equal to /KK. in thevalue of R.

The resistance Th can be subdivided into a number of parts forming asmany separate sensing elements each having its own collector anddistributed over the area to be supervised and connected either inparallel or in se ries in the same circuit. The rest temperature of allthe thermistors is therefore substantially identical. If a fire starts,one or more sensing elements will be affected and their resistance willdecrease, While the resistance of the unaffected elements does notchange. The presence of the inactive elements weakens the signaldelivered by the affected elements. All the sensingelements must, ofcourse, be installed in the same thermal surroundings to ensure that therest temperature (and therefore the inoperative resistance) of all nsensing elements is substantially the same. The sensitivity of theindicating or warning circuit can therefore be increased so that asignal delivered by in affected sensing elements is enough to initiate aWarning signal although m-n elements remain inactive.

volts/degree=XE(K)dt (4) This leads to a slight change These sensingelements, being at the same rest temperature in common thermalsurroundings, will hereinafter he referred to as thermally dependent.

In practice, an absolute temperature is bound to be exceeded at a lowrate of temperature rise, otherwise the detector circuit would alreadyhave responded. It can therefore be assumed that when the criticaltemperature is reached, all the sensing elements are at substantiallythe same temperature. Hence, since all the thermistors behave in unisonas a single thermistor, the critical level will be exceeded nearly atthe same temperature, however many sensing elements are connected to thewarning circuit.

Summing up, the differential sensitivity of a detector circuitcomprising one or more thermally dependent sensing elements is usuallyless than the differential sensitivity of a similar circuit comprisingonly one sensing element subjected to the same heat source, although thedecrease in sensitivity is independent of the rest temperature of thearrangement. On the other hand, the sensitivity of an absolutetemperature detector circuit is little affected by the number ofelements it comprises, provided that they all are mounted in the samethermal surroundingsi.e., that they are thermally dependent.

Conventionally, the sensitivity of a fire detector is defined as theminimum rate (assumed to be constant) of temperature rise required for awarning signal to be given within a specified time, usually 30 to 60seconds. Of course, this assumption of linear temperature variation isnot found in practice but is a convenient bases for comparing thesensitivity of various detectors and makes it possible to state theexact technical requirements of a particular detector circuit. As arule, practical values of sensitivity lie between 2 and l5/minute. Itwill be assumed hereinafter that the ambient temperature Ta varieslinearly in time. If the collector mass is low, if its thermalconductivity is high and when the thermal coupling with the sensitiveelement is efficient, the temperature T thereof varies in substantiallythe same Way after an initial period (FIG. 5). It therefore follows thatthe electric signal delivered at M by the thermistor in the circuitshown in FIG. 3 is also substantially linear. This signal is applied tothe differentiating circuit.

H. Simple Detector Circuit (FIG. 6)

This term denotes an assembly comprising the sensing circuit hereinbefore described and now associated with a specially-designeddifierentiating circuit.

The same comprises a condenser C connected in series with a resistance1' through which flows a current i supplied through a diode D.

The diode and the constant current it delivers will temporarily beneglected. The inoperative potential of the point X is therefore that ofthe positive pole of the battery. If a voltage which increases linearlyat a rate of k volts/ second is applied to the condenser C, the currentflowing through the resistance r is a function which is a solution ofthe following differential equation of equilibrium:

ri+l/Cfidt=kt There are two possible cases:

(1) The voltage k increases during a period which is short or at themost of the same order of magnitude as the time constant ;-C of thecircuit and therefore remains constant. The current i therefore variesin accordance with two segments of exponentials which increase anddecrease consecutively with the time constant of the circuit.

(2) The voltage k increases during a period which is long relatively tothe time constant 1C, and then remains constant.

In this case, the current curve takes the form of a pulse, the flanks ofwhich are segments of exponentials, while the final amplitude is kCamperes (FIG. 8).

8 A complete differentiating circuit is shown in FIG. 6 and will now beconsidered. It will be assumed that the diode is connected to thepotential E". Therefore, and in the absence of any signal at M, thepotential of the point X is nearly E", while an inoperative currentflows through the resistance r. The value of E must be less than thelowest potential which can be assumed by the point Mi.e., at the highesttemperature to which the thermistors may raise; for the condenser C,being of a relatively high capacity, is of the electrolytic kind andshould therefore always have a voltage applied across it in the samedirection. As will become apparent hereinafter, the best arrangement isthat in which the negative pole is connected to X. This value of 15 istherefore fairly low in practice, for example, 10-20% of E.

If a linearly decreasing potential is applied to M due to linearincreasing of the temperature of Th, Formula 6 shows that the condenserdelivers to the resistance r a current of the kind shown in BT68. 7 or8.

Provided that the latter current remains less than (EE)/r, nodifferential signal appears at the point X which remains at the restpotential. On the other hand, if the condenser discharge current rises,the point X will tend to become more negative and the potential of thepoint X will vary along an exponential path of the kind shown in FIG. 9.It will he noted that when the potential of X has dropped a little, thediode D is biased to cut off and a supplementary reverse current i whichis constant and, if the diode is correctly chosen, very low, is takenfrom the condenser. Such current accelerates the discharge of C in theproportion of i /C volts/ second. However, the effect of the diode D canbe eliminated if a signal of i C volts/ second is subtracted from thetotal signal delivered by the thermistor at M. Similar considerationsapply if a diode d is connected, for a purpose to be describedhereinafter, across the condenser C in the reverse direction. If thecurrents of these diodes are i and i respectively, the effect of thediodes can be eliminated by subtracting the current (i +i )/C from thetotal signal, and the net available signal will hereinafter beconsidered as k(i +i )/C:k' volts/ second. The minimum variations of thepotential of M required to produce a differentiated signal of a specificlevel is therefore greater when the presence of the diodes is taken intoaccount. Since the reverse current of the diodes istemperature-dependent, the diodes will be such that their maximumreverse current is low relative to the mean current absorbed by r.

In some extreme cases Where the temperature is very high, silicon diodesmay be used, in which case It substantially=k.

Summing up, upon the receipt of any incident signal of sufficientvariation the normally energized differentiating circuit hereinbeforedescribed delivers an exponential differentiated signal if k'C isgreater than i, the latter signal having an asymptotic level of kCrvolts.

The dilferentiating circuit therefore acts after the fashion of avariable signal limiter and detector, the detector having a fixed andarbitrarily adjustable threshold (adjustable by the level of i). Thedifferentiating circuit therefore rejects any incident signal having avariation below a specified minimum. In some special cases it may beconvenient to connect the resistance r, not to the positive pole of thebattery, but to a potential E between E and E (FIG. 12).

The marginal sensitivity of a detector circuit can therefore be definedby a careful choice of i and by acting on E (or E 13 and r.

The time constant of the differentiating circuit is determined byappropriate dimensions of C and r. A suitable combination of thesevalues will provide different warning criteria which can be adapted toparticular protection problems.

g. The signal available at X is finally supplied to an indicatingamplifier.

III. Simple Indicating or Warning Circuit This circuit serves to showthat'a warning signal has been initiated.

It comprises a transistorised current emplifier supplying a polarizedrelay P as shown in FIG. The relationship between the current deliveredby the amplifier and the voltage applied to the input-i.e., across thebase of Ta and the emitter of Tb-is shown in FIG. 11 which also showsthe operating current limits of the polarized relay. The relay isenergized at a clearly-defined input voltage. The relay has an armaturewhich can take up two operating positions and one neutral position. Theamplifier provides a gain of from 2000 to 4090. If the relay P issensitive and can operate, for instance, on 300' to 400 microamperes,the relay will be energized upon receipt of an input signal of littlemore than a few hundred millivolts and with an input current of asmaller fraction of a milliampere. If the circuit shown in FIG. 10 isassociated with the circuit shown in FIG. 6, the damage alarm indicationcircuit shown in FIG. 12. is produced. Since the current i=:(E E)/r ismuch greater than the current consumed by the amplifier even at theinstant when the relay operates, the amplifier current and the presenceof the amplifier can be neglected. The potential at X therefore variesas shown by the curves previously explained, and the appearance of asignal at X depends solely upon the factor k' i/Ci.e., upon whether therate of temperature rise is high as compared with the maximum normalrise corresponding to i".

The circuit constants are so chosen that the net signal reaches thecritical value krC of 200 mv. within a given time for a given excess ofrate of temperature rise maintained for such given time. For instance,no appreciable net signal is produced by a rise of 3/rninute, whereas awarning is initiated within 40 seconds by an increase of 40/minute;i.e., by an excess rate of rise of only 1/minute maintained for 1minute.

If the linear increase of the signal at M is maintained indefinitely,the potential at X decreases along an elongated exponential path(according to the time constant rC) tending towards the value krC. Thetime constant is adapted to the detection conditions required by theuser.

It will be apparent from the foregoing that in the present case It iswhen DT/dt=l C./minute. The factors 1', C, K, E" can always be combinedto provide the required sensitivity. When the thermistor cools, theincrease of potential at M causes the condenser C to be rechargedthrough the diode D and resistance r (FIG. 12). Since'the value of thisresistance is much lower than that of r, being, for instance,one-thousandth of r, the time constant r C is very low (for instance,less than one second), so that the potential at Y is scarcely affectedby the charging current. Hence, when the sensing element is cooled, thecircuit is adapted without delay to the new temperature conditions.

The circuit always provides the condenser C with a charge correspondingto sensing element temperature, whether the same is constant or slowlyrising or rapidly dropping. It will be readily apparent that thedetector circuit is therefore fully sensitive at any time and operateswithout any delay to show that there has been an abrupt and unexpectedtemperature rise during cooling of the area under supervision.

The indicating or warning circuit is also required to provide a separatetechnical incident indication of any complete or partial failure of adetector circuit. These faults are either the short-circuiting, earthingor accidental opening of one or more sensing elements in any wayassociated with the circuit. Whatever the fault, it is seen by the pointM as an abrupt potential variation in one or the other direction (FIG.13). Since the variation is abrupt, it is transmitted withoutattenuation to the point X to which two technical incident indicationtransisors T and T are connected through condensers c and c (FIG. 13).As can be seen in FIG. 13, the latter transistors must be ofcomplementary polarity, since technical warning signals may be of eitherpolarity. The transis'tors are operated by the differentiating circuitsformed by r 0 and r 6 respectively. The time constants r 0 and r 0 ofthese circuits are very low so that the current produced even by maximumpotential variation at X due to heating of the thermistors isinsufficient to open the transistors. These amplifiers therefore remaininoperative, in particular forany fire alarm indication. On the otherhand, a technical incident signal, even if of low amplitude, istransmitted with its complete initial amplitude and applies to thetransistors a large exponential signal. Nevertheless, such signal lastslong enough to operate one of the corresponding technical incidentsalarm relays AT or AT shown in FIG. 13.

The low time constants of the circuits associated with the transistors Tand T are produced by reducing c and c for instance, to one-tenth orone-twentieth of C. 1- and r are adjusted to give a time constant of,for instance, to 200 milliseconds.

Of course, the condenser 0 reduces the net fire alarm signal availableat X due to the current which it draws because of the potentialreduction at the point X. It can readily be shown that, if the timeconstant r c is small relative to rC, as is the case in practice, thepresence of 0 leads to an imaginary reduction of C in the ratio C/(C-t-cwhich means in practice that the capacity of C is reduced by 10%.

It will be noted that theamplifier receives a fire alarm signal if asensing element is accidentally short-circuited. Such. an unreal alarmsignal must be neutralised.

To this end, the winding I of the polarized relay is shunted by anelement representing a voltage threshold Z of a level such that themaximum current normally required to energize the relay produces avoltage drop slightly below said threshold (FIG. 13). On the other hand,the technical incident signal just considered produces in the sameWinding a current limited to a value slightly above the requiredenergizing current, the excess current being absorbed by the element ZThe relay would therefore tend to give a fire alarm, were it not for thefact thatall the current delivered by the technical incident amplifierflows through a second and opposing winding II. The winding II is suchthat the opposing ampere-turns predominate so that the polarized relayis immediately operated in the sense of a technical alarm. A diagramillustrating these principles is given in FIG. 14.

In addition to the elements already described, there can be seen arectifier bridge B for enabling either of the amplified technicalincident. signals delivered by T or T to be applied to the winding II ofthe polarized relay P. The fire alarm or technical incident signals arekept on permanently by relays RAS and RAT respectively which latch tooperate the suitable warning signal permanently.

Any accidental shorting of the condenser C is indicated as a technicalalarm due to the abrupt and considerable rise in the potential at X.This is one of the main reasons why the potential of E is set below thelowest possible potential of the point M. y

In the foregoing a description has been given of the manner in which thealarm circuit is operated by an excessixe rate of temperature rise. Ifrequired, an alarm can also be operated for any excess of an absolutetemperature considered dangerous. To this end a diode D is connectedacross the condenser C with its cathode connected to the positiveterminal. of C.

The value of Th (or of R) should be such that, when the criticaltemperature is reached, the additional reduction in the resistance of Thdecreases the potential at M to below E", so that the diode D becomesconductive and the potentials at M and X act in unison and vary togetherupon any further temperature rise. Hence the slightest excess of thecritical temperature produces at X a signal of sufficient amplitude toopen the transistorized amplifier and operate the relay P to give a firealarm warning. It will be noted that the diode D is biased in theopposite direction for any temperature of Th below the critical figure,for the potential of M is greater than that of E. If the reversesaturation current of the diode D is negligible, the operation of thecircuit as a velocirnetric detector is not affected by the presence ofthe latter diode. If required, D can be a silicon diode, in which caseit requires a minimum voltage of about 0.4 v. to become conductive, sothat a suitable value of Th (or R) must be chosen.

There is no disadvantage in the condenser C becoming slightly polarizedin the opposite direction when D becomes conductive.

Voltage-step elements Z, Z and Z are provided to stabilize the voltage Eand E +e despite variations of the battery voltage, for the least suchvariation would produce a fire alarm or technical incident signal at X.

A protective resistance r limits the base current of Th to a safe value.r is low enough not to cause a harmful voltage drop in the fire alarmsignals applied to the transistor.

The resistances 2' and r stabilize the inoperative current of the basesof T and T in the absence of any signal. The shunt diodes discharge thecondensers c and c after a technical incident alarm has been given.

The device hereinbefore described forms a complete alarm circuit whichcan be operated by one or more sensing elements installed in the samethermal surroundings and connected to a single sensing circuit. Ofcourse, the sensitivity of each sensing element operated individually isless than the sensitivity which the same sensing element Would havealone in the circuit (with the value K=I at the means temperature). Thisreduction in individual sensitivity becomes greater in proportion asmore elements are associated in a single circuit. As has been shown,this reduction in individual sensitivity is not a disadvantage, providedthat the associated sensing elements are disposed in the same thermalsurroundings, for an outbreak of fire would affect a number of adjacentsensing elements simultaneously and their individual signals would besuperimposed upon one another to provide a high-amplitude alarm signal.

However, in some cases a sensing element may be required to retain itsindividual sensitivity although installed with other elements in thesame room. To this end, each sensing element can be provided with anindividual differentiating circuit and the complete arrangement can beconnected to a common indicating or warning circuit. The connectionthereto is such that the behaviour of any detector circuit is notaffected by the thermal condition of the others, just as if the detectorwas alone in the circuit. Sensing elements thus installed and theassociated detector circuits will hereinafter be referred to as thermally independent. It will be apparent that, even if a considerablenumber of thermally independent sensing elements are provided in oneroom, there is no need to identify the element which responded to anoutbreak of fire, for the same can be immediately located by identifyingthe common site of the sensing elements associated with the samecircuit.

Substantially any number of identical sensing circuits are connected inparallel with the voltage E (FIG. 15). Each of these circuits of order khas its own differentiating circuit r C The detector circuits thusformed are connected to a complete indicating and warning circuitsimilar to the kind hereinbefore described by means of two groups ofdiodes S and G each forming an or-gate. Two examples, k and k+I only, ofsuch detector and differentiating circuits are shown.

The diodes Si are silicon diodes. The resistance r,. and

12 the diodes D, G G etc. at each connected across a resistance rmaintain the diodes Si when the circuit is at rest, at a potentialslightly below the potential on their voltage-current characteristicswhere they start to become conductive (FIG. 15).

The complete circuit operates as follows (FIG. 15): if a sensingelement, for instance Th is heated at a rate greater than the criticalrate, the potential drops at M and X as hereinbefore described, so thatthe diode G is cut off and the diode Si is rendered conductive. Thetransistors T and T are, therefore, in turn opened and the alarm relay Pis quickly operated to give a fire alarm. Similiarly, when a sensingelement Th is ruptured or short-circuited, the abrupt potentialvariation at M and X opens the diode G or the diode Si respectively, sothat the potential variation of M is applied either to the transistor Tor the transistor T and the relay P operates to give a technical alarm.

When the diode Si is operated, it biases the 12-1 passive diodes Si ofthe gate and the diode G to cut-off. Hence the reverse current of thesenl diodes, since it is applied in toto to the differentiating circuit rC must be greatly reduced, whereas the reverse current of the diode Dcan be greater, provided that it is a small fraction of the currentabsorbed by r The potential at X or Y, therefore, depends only upon therate of change of the potential at the places M of the detector circuitoperated; i.e., upon the rate of ambient temperature change, just asassumed in the theory of the simple detector circuit.

IV. Identifying and Marking Circuits These circuits identify and locatedetector circuits in which one or more sensing elements have set off analarm or have gone out of order.

Detector devices with individual indications of the kind shown in FIG.14 provide, by their very nature, an individual alarm, the reason forwhich can be traced. This is not the case, however, with the detectorshaving central indicating circuits of the kind shown in FIG. 15, Wherealarm signals are essentially anonymous. As previously stated, sensingelements connected to the centralized indicating circuits must be placedin the same room.

The centralized indicating circuit can, however, be adapted to caseswhere detector circuits are to be fitted in different places and arethermally independent such as, for instance, in hotel rooms, enclosedplaces at public performances, etc. In this case the central indicatingcircuit is supplemented by as many individual identifying and markingcircuits as there are centralized detector circuits to be identified andby an automatic connection circuit.

The circuit to be described hereinafter and illustrated in FIG. 16utilizes the centralization possibilities of the indicator described insection III hereof yet provides immediate identification of the detectorcircuit concerned and of the nature of the alarm (fire or technical).

FIG. 16 is given by way of example and is not in itself a featurelimiting the scope of the invention.

It will be seen that in FIG. 16 the circuit shown in FIG. 15 isassociated with a number of indicating and marking circuits individuallyassociated with n detector circuits. In FIG. 16 the illustration ofthese individual circuits is limited to the circuit associated with thedetector circuit k. It comprises fire alarm and technical incidentrelays RAS; and RAT, supplied through transistors T and T respectively.There are therefore as many pairs of relays and transistors as there aredetector circuits to be individualized. These individual circuits areconnected to the central indicating device by connecting relays Co. Thisrelay comprises as many change-over contacts as there are circuits to beindividualized. Two general failure and fire arm relays RAS and RATassociated jointly with all the individual detector circuits completethe circuit. The latter relays operate when any detector circuit hasinitiated an alarm and ensure that the alarm is given permanently 13 bylatching. An acoustic warning (not shown) is then operated permanently.

The circuit operates as follows:

When the circuit is in the rest-state, all the relays are released andare in the position shown in FIG. 16. The detector circuits arethereforeconnected to the central indicating circuit in the same way as in FIG.15.

If a detector. circuit of identity k initiates a fire alarm, the relay Pis operated as hereinbefore described and in turn operates an auxiliaryrelay P which in turn energizes the connecting relay CO. The sameimmediately interrupts the connection between the condensers C (k=1 n)to the gates formed by the diodes Si and G and connect each of themindividually to the transistor pair T andT (k l 21) associated with eachdetector circuit. Sincethe transistor emitters are connectedto apotential close to E", the new connection made by C does not alter thebase potentials of transistors associated with circuits not giving afire alarm. On the other hand, the detector circuit which has operatedthe relay P opens the transistor T associated therewith so that thecorresponding individual damage alarm relay M8,; is operated. The samelatches and gives a light warning at position k of the individual alarmpanel (not shown). Also, the relay RAS immediately connects thecondenser C to the potential E". Finally, it operates the general firealarm relay RAS to start a permanent damage audiblewarning.

However, once the relay Co has operated, the negative side of C isdissociated from the potential at X, and sothe relay P and therefore therelay -P falloff. Since the relay P is shunted by a diode,its-falling-off is fairly delayed, so that the delay C0 can remain ondespite the disappearance of the signal from X. Thelatter relay is alsoprovided with a diode-for delaying its falling-off so that the signalproduced at X is kept applied to the bases of T and T longenough fortherelay RAS to operate and'latch on. Since the alarm signal at X isapplied to the only conductive-transistor T -(instead of'the two seriesconnected transistors T and T the current flowing through the basethereof is considerable and is sufficient for the transistor T tooperate the individual alarm relay despite the accelerated discharge ofthe condenser C Finally, P, P and CO fall off due to the operation andlatching of RAS and RAS. This means that the detector circuit giving afire alarm has been identified, marked and cut out of operation, thatits alarm condition has been remotely indicated by an audible warningand that the centralized general alarm circuit is reconnected to theremaining n-l operative detector circuits.

It will now be assumed that the identity detector lc has just beenshort-circuited. The relay P therefore gives a technical alarm asdescribed with reference to FIG. 15. The auxiliary relay P is operatedand in turn operates a connecting relay Co. The same operates as in thecase just described so that the relay RAT is operated through the agencyof the transistor T (to which it has been connected by a make-contact ofthe connecting relay P and the general technical incident alarm relayRAT operates and latches on to provide an appropriate permanent alarm,while the detector circuit of order k is eliminated and the centralindicating circuit is reconnected to the remaining operative n1detectors.

Finally, it will be assumed that the identity detector circuit k hasjust been opened. The process is exactly as in the case just described(except that the transistor T operates the relay RAT and leads to thesame alarm relays being operated as were considered in the previouscase.

In short, the detector circuits have sensing elements which arecompletely static, strong, unchanging, free from any electrical contactand of reduced dimensions. These static elements deliver a signalinterpreted by an RC- circuit biased by an inoperative current. Thelatter circuit defines the maximum admissible rate of temperature riseor the velocimetric sensitivity of the detector circuit. Also, it maydefine an absolute temperature level which must not be exceeded; i.e.,the thermometric'sensitivity; These sensitivities can readily beadjusted independently of one another. Any'desir'ed number of sensingelements can be associated with the same detector circuit. Simi larly,any desired number of detector circuits can be associated with the samecentral indicating or warning circuit, yet the individual sensitivitiesof such circuits remain independent and individually adjustable. Suchcentralization doesnot impair identification of the sensing circuitwhich has operated. The detector circuits are supervised continuously tosee that they are doing their job. Any fault in anyof them is shownup,'an'd thefaulty circuit is. identified, marked and eliminated and'apermanent special alarm is given. The consumption of all the detectorand indicating circuits is greatly reduced so that small batteries canbe used. Semiconductors make the circuits very stable. Highly reliableoperation is ensured by using hermetically sealed polarized relays,while the use of sealed batteries supplied at a constant voltage reducesmaintenance costs to a very low figure;

The device differs from known devices in that the sensing elements arevery'small, static, sturdy and free of electric contacts and in that asecond and insulated thermal element'is not required, slow thermalchanges being compensated for by electrical means. Also, and in contrastwith known devices, the velocimetric sensitivity and the thermometricsensitivity are defined by a single indicating or warning circuit whichinterprets the state of a single sensing element, or of a group ofassociated sensing elements, on essentially electrical bases.

In contrast to known devices, a considerable number of detector circuitscan be associated with one central indicating circuit, yet the detectorcircuits remain independent of one another and the circuit giving thealarm or having the defect can stillbe identified.

Finally, and in contrast to mostof the known circuits, electronicdetectors consume very little current.

I have described what I believe to be the best embodiments of myinvention. I do not wish, however, to be confined to the embodimentsshown, but what I desire to cover by Letters Patent is set forth in thefollowing claims.

What I claim is:

1. A circuit for detecting a rate of temperature change, comprising asource of direct voltage, a first resistor, a sensing element having atemperature-dependent coefficient of resistance series connected at afirst junction with said first resistor across said voltage source, asecond resistor connected at one end to said voltage source, a normallyforward biased unilateral conducting means having one side tied to apoint of constant potential and another side connected at a secondjunction to the other end of said second resistor for normallymaintaining a constant flow of current through said second resistor,capacitive means connected between said first junction and said secondjunction to block the passage of DC. therebetween and to produce asecond current through said second resistor in response to a change ofresistance of said sensing element, said second current flowing in adirection to produce a voltage drop across said second resistor tendingto cut off said unilateral conducting means, and output means connectedto said second junction for detecting signals produced by predeterminedrates of temperature change, whereby upon a predetermined rate oftemperature change in a given direction said unilateral conducting meansis biased off to disconnect said constant potential point from saidoutput means.

2. A circuit according to claim 1, including a semiconductor amplifier,a relay connected to the output of said amplifier, the input of saidamplifier being connected to said second junction a difierentiatingcircuit having a low time constant relative to the temperature changes,and a fault detector, one end of said difierentiating circuit connectedto said second junction and the other end of said differentiatingcircuit connected to said fault detector, said amplifier beingresponsive to predetermined voltages at said second junction.

3. A system for detecting excess rates of temperature rise comprising asource of direct voltage, a plurality of detector circuits connected inparallel, each detector circuit including a first resistor connected atone end to the positive side of said voltage source, a sensingthermistor connected at one end to the other end of said first resistor,at a first junction, the other end of said thermistor being connected tothe negative side of said voltage source, a capacitor connected at oneend to the junction of said first resistor and thermistor, a secondresistor connected at one end to the positive side of said voltagesource, a first diode, a second diode, the anode of said first diodebeing connected to the other side of said second resistor, the other endof said capacitor and the cathode of said second diode, a constantpotential resistor connected at one end to the cathode of each of saidfirst diodes in said plurality of detector circuits, the other end ofsaid constant potential resistor being connected to a point of constantpotential, a detector means connected to the anode of each of saidsecond diodes in said plurality of detector circuits, whereby thesensing of a predetermined excessive rate of temperature rise by any ofsaid thermistors actuates said detector means.

4. A system according to claim 3 including a pnp transistor and an npntransistor, a first predetermined small time constant RC circuitconnected to the base of said pnp transistor the anodes of said seconddiodes being connected to said first RC circuit, a second predeterminedsmall time constant RC circuit connected to the base of said npntransistor the cathodes of said first diodes being connected to saidsecond RC circuit whereby a malfunction in the sensing circuit producesa signal which is sensed by one of said transistors.

5. A circuit for detecting an excess rate of temperature rise comprisinga source of direct voltage, a first resistor connected at one end to thepositive side of said voltage source, a sensing thermistor connected atone end to the other end of the first resistor at a first junction, theother end of said thermistor being connected to the negative side ofsaid voltage source, a capacitor connected at one end to the junction ofsaid first resistor and thermistor, a second resistor connected at oneend to the positive side of said voltage source, a diode having itscathode connected to a point of constant potential and its anodeconnected to the other side of said second resistor and the other sideof the capacitor at a second junction, the circuit parameters beingdimensioned so that a stand-by current flows in said second resistor andsaid diode during decreasing temperatures, constant temperatures andtemperature rate increases below a predetermined level, said diode beingcut off when a rate of temperature increase in excess of said level isdetected by said thermistor, detecting means connected to said secondjunction whereby a detection signal is produced at the anode of saiddiode, a third resistor con nected to the cathode of said diode, theother end of said third resistor being connected to the positive side ofsaid voltage source, a pnp transistor, a first predetermined small timeconstant RC circuit connected at one end to the anode of the diode andthe other end of said first RC circuit connected to the base of the pnptransistor, an npn transistor, a second predetermined small timeconstant RC circuit connected at one end to the cathode of the diode andthe other end of said second RC circuit connected to the base of the npntransistor whereby a malfunction in the sensing circuit produces asignal which is sensed by one of said transistors.

References Cited in the file of this patent UNITED STATES PATENTS2,742,634 Bergen Apr. 17, 1956 2,827,624 Klein Mar. 18, 1958 2,901,740Cutsogeorge Aug. 25, 1959 3,038,106 Cutsogeorge et a1. June 5, 1962

1. A CIRCUIT FOR DETECTING A RATE OF TEMPERATURE CHANGE, COMPRISING ASOURCE OF DIRECT VOLTAGE, A FIRST RESISTOR, A SENSING ELEMENT HAVING ATEMPERATURE-DEPENDENT COEFFICIENT OF RESISTANCE SERIES CONNECTED AT AFIRST JUNCTION WITH SAID FIRST RESISTOR ACROSS SAID VOLTAGE SOURCE, ASECOND RESISTOR CONNECTED AT ONE END TO SAID VOLTAGE SOURCE, A NORMALLYFORWARD BIASED UNILATERAL CONDUCTING MEANS HAVING ONE SIDE TIED TO APOINT OF CONSTANT POTENTIAL AND ANOTHER SIDE CONNECTED AT A SECONDJUNCTION TO THE OTHER END OF SAID SECOND RESISTOR FOR NORMALLYMAINTAINING A CONSTANT FLOW OF CURRENT THROUGH SAID SECOND RESISTOR,CAPACITIVE MEANS CONNECTED BETWEEN SAID FIRST JUNCTION AND SAID SECONDJUNCTION TO BLOCK THE PASSAGE OF D.C. THEREBETWEEN AND TO PRODUCE ASECOND CURRENT THROUGH SAID SECOND RESISTOR IN RESPONSE TO A CHANGE OFRESISTANCE OF SAID SENSING ELEMENT, SAID SECOND CURRENT FLOWING IN ADIRECTION TO PRODUCE A VOLTAGE DROP ACROSS SAID SECOND RESISTOR TENDINGTO CUT OFF SAID UNILATERAL CONDUCTING MEANS, AND OUTPUT MEANS CONNECTEDTO SAID SECOND JUNCTION FOR DETECTING SIGNALS PRODUCED BY PREDETERMINEDRATES OF TEMPERATURE CHANGE, WHEREBY UPON A PREDETERMINED RATE OFTEMPERATURE CHANGE IN A GIVEN DIRECTION SAID UNILATERAL CONDUCTING MEANSIS BIASED OFF TO DISCONNECT SAID CONSTANT POTENTIAL POINT FROM SAIDOUTPUT MEANS.